The containment and elimination strategy for SARS-CoV-2 will require long-term prevention measures in the entire population.

By introducing a model that reveals long chains of SARS-CoV-2 transmission and inadequate passive surveillance systems, a new medRxiv * The pre-printed paper shows that bringing the infection fully under control will have to rely on presumptive, stringent, longer and more costly prevention measures for the entire population already employed against community transmission.

As the world continues to grapple with the disruptive global pandemic of coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it is critical to understand why the causative agent of the virus original The SARS outbreak (i.e. SARS-CoV-1) followed a dramatically different epidemic trajectory.

Despite the very similar rapid spread of nonspecific pathogens and symptoms, SARS had a significantly higher mortality rate; However, the epidemic curve for SARS-CoV-1 rapidly reversed, with only 774 deaths before the primary outbreak ended in July 2003.

The decreasing probabilities over time, expressed in terms of the total duration of viral generation (g), each last approximately two weeks but overlap approximately one week apart for SARS-CoV-1 and SARS- CoV-2,^two for transmission chains seeded by individual individual primary cases remain unconfirmed by routine passive surveillance of self-reported symptomatic cases (P_{uc, g}), assuming a range of values ​​for the effective reproductive number in the absence of any contact tracing and isolation intervention (R_me). The horizontal dashed lines in both panels represent a 50% probability, from which the mean chain length can be interpolated on the horizontal axis.

So on the one hand, we have the original SARS outbreak decisively eradicated, while the COVID-19 pandemic has spiraled out of control. Even more worrying, why has COVID-19 proven so difficult to eliminate locally, even in countries that managed to reduce their national outbreaks to just a handful of residual cases?

While the transmission of presymptomatic diseases is believed to be a key feature of the epidemiology of SARS-CoV-2, primarily by facilitating spread to new settings, this alone does not automatically prevent the tracing and isolation of effective contacts. Instead, it exacerbates the need to do so quickly and with a greater retrospective scope.

In a new pre-printed article, Dr. Gerry F. Killeen from the Faculty of Biological, Earth and Environmental Sciences and the Institute for Environmental Research at University College Cork, Ireland presented a simple deterministic model to describe how SARS infections -CoV-2 results in long chains of transmission lasting several weeks undetected through conventional, self-report passive surveillance efforts.

A schematic illustration of how (A) chains of SARS-CoV-1 transmission can be consistently contained by tracking and isolating effective contacts because the vast majority of infections result in clinical symptoms evident enough to motivate self-notification to health facilities or testing centers, while (B) for SARS-CoV-2, only a small minority of infections cause symptoms evident enough to motivate self-notification, so the chains Transmission lines are often spread too far before they are detected for tracing and retrospective contact isolation to contain them completely.

Modeling drive chains

In this model, new infections are assumed to fall into one of three categories of symptomatic outcomes, each associated with a well-defined probability of self-reporting to disease-specific clinical facilities or testing centers.

These categories are the proportion of people infected with severe symptoms at some stage of the infections (all reporting for testing), the proportion of people infected with mild symptoms during the course of the disease (sometimes reporting for testing), and the proportion of infected people without any obvious symptoms (not self-report for testing).

In addition, the reproductive reference number used in this document specifies an intervention scenario without contact tracing and isolation for confirmed cases identified through routine passive symptom-based surveillance in testing centers and health facilities.

Implications of the phenomenon

In any case, the long, thin chains of transmission of SARS-CoV-2 that make passive surveillance difficult have several implications according to this model. First, such surveillance is less sensitive as an indicator of infection-free status in the low reproductive numbers required to achieve elimination endpoints, which means that follow-up periods must be prolonged.

Likewise, reproduction figures should be kept as low as possible to ensure that lifting of restrictions does not result in a rebound. Still, we must bear in mind that the more extended chains of transmission at lower reproductive numbers are often too long to be traced back, making them underrepresented in surveillance data.

“This analysis also underscores the importance of understanding the large differences that local effective reproductive figures can have on the traceability of SARS-CoV-2 infections and the implications of these detection biases for the interpretation of surveillance data.” the study authors state in this medRxiv paper.

In cases where surveillance systems are weak or younger age groups with no obvious symptoms dominate transmission, the effectiveness of containment of isolation and contact tracing may be more severely restricted, even at the highest reproduction numbers. which are related to larger outbreaks.

Efforts of the entire population with ‘special ingredients’

“If in fact contact tracing and isolation is much less effective against slow, steady and undetected community transmission of SARS-CoV-2, it follows that containment and elimination efforts will have to rely predominantly on prevention measures. more onerous and presumptive for the entire population that have been shown to be so effective thus far, “the study authors conclude.

Simply put, compared to the measures in place for SARS-CoV-1, COVID-19 suppression efforts will need to be held at a very strict level and for extended periods of time after the last detected case of community transmission.

Finally, given the current lack of alternatives, the most important ingredients that will be necessary for many countries to move towards eliminating the transmission of SARS-CoV-2 are political will, ambition, perseverance, patience, and a broad public support.

*Important news

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.